Traditionally, digitally controlled color printing capability is accomplished by one of two technologies. Both require independent ink supplies for each of the colors of ink provided. Ink is fed through channels formed in the printhead. Each channel includes a nozzle from which droplets of ink are selectively extruded and deposited upon a medium. Typically, each technology requires separate ink delivery systems for each ink color used in printing. Ordinarily, the three primary subtractive colors, i.e. cyan, yellow and magenta, are used because these colors can produce, in general, up to several million shades or color combinations.
The first technology, commonly referred to as “droplet on demand” ink jet printing, selectively provides ink droplets for impact upon a recording surface using a pressurization actuator (thermal, piezoelectric, etc.). Selective activation of the actuator causes the formation and ejection of a flying ink droplet that crosses the space between the printhead and the print media and strikes the print media. The formation of printed images is achieved by controlling the individual formation of ink droplets, as is required to create the desired image. Typically, a slight negative pressure within each channel keeps the ink from inadvertently escaping through the nozzle, and also forms a slightly concave meniscus at the nozzle helping to keep the nozzle clean.
Conventional droplet on demand ink jet printers utilize a heat actuator or a piezoelectric actuator to produce the ink jet droplet at orifices of a print head. With heat actuators, a heater, placed at a convenient location, heats the ink to cause a localized quantity of ink to phase change into a gaseous steam bubble that raises the internal ink pressure sufficiently for an ink droplet to be expelled. With piezoelectric actuators, a mechanical force causes an ink droplet to be expelled.
The second technology, commonly referred to as “continuous stream” or simply “continuous” ink jet printing, uses a pressurized ink source that produces a continuous stream of ink droplets. Traditionally, the ink droplets are selectively electrically charged. Deflection electrodes direct those droplets that have been charged along a flight path different from the flight path of the droplets that have not been charged. Either the deflected or the non-deflected droplets can be used to print on receiver media while the other droplets go to an ink capturing mechanism (catcher, interceptor, gutter, etc.) to be recycled or disposed. U.S. Pat. No. 1,941,001, issued to Hansell, on Dec. 26, 1933, and U.S. Pat. No. 3,373,437 issued to Sweet et al., on Mar. 12, 1968, each disclose an array of continuous ink jet nozzles wherein ink droplets to be printed are selectively charged and deflected towards the recording medium.
U.S. Pat. No. 3,709,432, issued to Robertson, on Jan. 9, 1973, discloses a method and apparatus for stimulating a filament of working fluid causing the working fluid to break up into uniformly spaced ink droplets through the use of transducers. The lengths of the filaments before they break up into ink droplets are regulated by controlling the stimulation energy supplied to the transducers with high amplitude stimulation resulting in short filaments and low amplitudes resulting in long filaments. A flow of air across the paths of the fluid at a point intermediate to the ends of the long and short filaments affects the trajectories of the filaments before they break up into droplets more than it affects the trajectories of the ink droplets themselves. Thus, by controlling the lengths of the filaments, the trajectories of the ink droplets can be controlled, or switched from one path to another. As such, some ink droplets may be directed into a catcher while allowing other, selected ink droplets to be applied to a receiving member.
U.S. Pat. No. 6,079,821, issued to Chwalek et al., on Jun. 27, 2000, discloses a continuous ink jet printer that uses actuation of asymmetric heaters to create individual ink droplets from a filament of working fluid and to deflect those ink droplets. A printhead includes a pressurized ink source and asymmetric heaters, operable to form printed ink droplets and non-printed ink droplets. Printed ink droplets flow along a printed ink droplet path ultimately striking a print media, while non-printed ink droplets flow along a non-printed ink droplet path ultimately striking a catcher surface. These non-printed ink droplets are then recycled or disposed of through an ink removal channel formed in the catcher. While the ink jet printer disclosed in Chwalek et al. works extremely well for its intended purpose, using the asymmetric heater to create and deflect ink droplets increases the energy and power requirements of this device.
In U.S. Pat. No. 6,851,796, which issued on Feb. 8, 2005, an ink droplet forming mechanism selectively creates a stream of ink droplets having a plurality of different volumes traveling along a first path. An air flow directed across the stream of ink droplets interacts with the stream of ink droplets. This interaction deflects smaller droplets more than larger droplets and thereby separates ink droplets having one volume from ink droplets having other volumes.
As the drop selection mechanism described above depends on drop size, it is necessary for large-volume droplets to be fully formed before being exposed to the deflection air flow. Consider, for example, a case where the large-volume droplet is to have a volume equal to four small-volume droplets. It is often seen during droplet formation that the portion of the ink stream that is to form the large-volume droplet will separate from the main stream as desired, but will then break apart before coalescing to form the large-volume droplet. It is necessary for this coalescence to be complete prior to passing through the droplet deflecting air flow. Otherwise the separate fragments that are to form the large-volume droplet will be deflected by an amount greater than that of a single large-volume droplet. Similarly, the small-volume droplets must not merge in air before having past the deflection air flow. If separate small-volume droplets merge, they will be deflected less than desired.
It has been found that the small-volume droplets between coalesced large-volume droplets can be very unevenly spaced. In extreme circumstances, the large-volume droplet often remains only partially formed until the large-volume droplet is well beyond the deflection air flow. The partially formed large-volume droplet and the small-volume droplet immediately in front of it must merge to produce the completed large-volume droplet. Occasionally, an undesirable merging of a small-volume droplet and a large-volume droplet will occur at some distance from the orifices. It is desirable to have the merging droplets coalesce as quickly as possible after break off without additional merging of the small-volume droplets with large-volume droplets or with adjacent small-volume droplets.